1. Field of the Invention
The present invention is directed to a method for producing carbon dioxide, nitrogen, and optionally argon, from a combustion exhaust gas. More particularly, the present invention is directed to a method for separating carbon dioxide from an oxygen depleted combustion exhaust gas to produce a feed gas enriched in nitrogen and argon.
2. Description of the Prior Art
The commercial preparation of carbon dioxide and nitrogen is well known in the art. Carbon dioxide is normally produced as a by-product from chemical processes for producing ammonia, hydrogen, ethanol, ethylene oxide, and gasoline, as well as in fermentation reactions and carbonate decompositions. Nitrogen is generally produced by separation from air.
The preparation of carbon dioxide generally involves the steps of crude gas generation, purification and separation, compression and liquefaction, drying, and rectification distillation.
Generation of crude carbon dioxide involves the combustion of liquid fuels such as fuel oil, or solid fuels such as anthracites, coke, charcoal, and the like, with excess air to promote complete oxidation of the fuel and to provide a carbon dioxide rich combustion exhaust gas.
Purification of the combustion exhaust gas generally involves several separate treatments to provide a gas having high purity. These purification treatments include washing, absorption, adsorption, desorption, and the removal of reducing substances. Washing generally involves a water absorption shower (water wash) to remove solids (soot, carried off ashes, etc.) and at the same time to cool the combustion gases. Various scrubbing solutions are generally employed to remove contaminants and to reduce the components in the combustion gas mixture to carbon dioxide, nitrogen, and oxygen. The combustion exhaust gas may also be passed through a tower containing a recirculating oxidizing solution such as potassium permanganate to remove traces of organic impurities carried with the gas.
The washed and scrubbed combustion gas is then separated to obtain a carbon dioxide rich fraction. In one separation method, the combustion gas mixture is circulated through a counter-current shower of an absorbing solution such as potassium carbonate, monoethanol-amine, and the like. Carbon dioxide can be desorbed by heating the carbon dioxide saturated solution to a temperature above 100.degree. C. In another separation method, the combustion mixture is separated by selectively adsorbing the carbon dioxide on a zeolite bed in a pressure swing adsorption system.
The purified and separated carbon dioxide is then compressed to a pressure in the range from about 230 psia to about 400 psia, dried by contacting the gas with a regenerable desiccant, and liquified by lowering the temperature of the gas. Finally, a rectification distillation step eliminates the small amount of nitrogen, oxygen, and argon to provide carbon dioxide having a purity of about 99.9% by volume.
The most common methods for separating nitrogen from air are cryogenic fractional distillation, inert gas generation (combustion of natural gas or propane in air), and pressure swing adsorption.
In cryogenic fractional distillation, air is compressed to about 100 psi and cooled in a reversing heat exchanger against outgoing nitrogen product gas and waste gas. Water, carbon dioxide, and hydrocarbons in the air are removed by condensation in the reversing heat exchanger. Alternatively, water, carbon dioxide, and hydrocarbons can be removed by passing air through a zeolite bed. The zeolite bed can be regenerated by passing heated nitrogen waste gas through the bed. The air is fed through a cold end gel trap where remaining small quantities of hydrocarbons and carbon dioxide are removed. The clean air is cooled further in a sub-cooler and is fed into a distillation column where the air is liquefied and separated into a high purity nitrogen product gas fraction and a waste gas fraction containing about 38% oxygen by weight. Both gas fractions are warmed to ambient temperature by passing the fractions through the sub-cooler and reversing heat exchanger.
In an inert gas generator, natural gas or propane is burned with air and the products of combustion are removed leaving purified nitrogen. The combustion of natural gas and air is controlled to provide a specific air to gas ratio in the burner to obtain essentially complete combustion. The combustion gas contains nitrogen, carbon dioxide, water vapor, and small amounts of carbon monoxide and hydrogen. Gases leaving the combustion chamber are cooled in a surface condenser to remove water. The gases then flow to a refrigerant dryer where the dew point is reduced to 4.degree. C. Pure nitrogen product gas is then obtained by passing the gas through a molecular sieve bed in a pressure swing adsorption apparatus to remove carbon dioxide and any remaining water vapor.
In a pressure swing adsorption system (PSA), air is passed at an elevated pressure through a bed of an adsorbent material which selectively adsorbs oxygen. Nitrogen product gas is then withdrawn from the bed. The adsorption bed may be regenerated by reducing the pressure of the bed.
U.S. Pat. No. 3,493,339, issued to Weir et al., discloses a method for producing carbon dioxide and separating argon which comprises combusting a carbonaceous material in a mixture of argon and oxygen and separating the combustion products to obtain carbon dioxide and argon.
U.S. Pat. No. 4,414,191, issued to Fuderer, discloses a pressure swing adsorption method for purifying hydrogen for ammonia synthesis. Nitrogen at elevated pressure is used as the purge gas in the pressure swing adsorption separation and the nitrogen in the purified gas is employed in the ammonia synthesis stream.
U.S. Pat. No. 4,797,141, issued to Mercader et al., discloses a method for obtaining carbon dioxide and nitrogen from the oxygen rich exhaust gas of an internal combustion engine or turbine. The method comprises the steps of cooling the exhaust gas, separating carbon dioxide from the cooled gas by absorbing the carbon dioxide in an alkaline solution, recovering the carbon dioxide by liberating the gas from the carbonated solution, compressing and liquifying the carbon dioxide, recovering the nitrogen by purifying the gas to remove contaminants, and compressing and liquifying the nitrogen.
While the above methods provide improvements in the production of carbon dioxide, none of these methods are entirely satisfactory. Conventional sources for producing carbon dioxide are carbon dioxide rich gases such as waste gases from ammonia, hydrogen, ethanol, and ethylene oxide plants. These carbon dioxide sources are not always available or are not always reliable especially at locations of high carbon dioxide demand. Other common problems with the production of carbon dioxide are low product yield and energy inefficient separation methods. Conventional gas generation methods do not teach the preparation of food grade carbon dioxide as well as pure nitrogen and argon from combustion exhaust gases. Hence there is a need for an improved method for producing carbon dioxide. The present invention provides such an improved method and also provides an improved method for producing nitrogen and argon as by-products.